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by Nick Knisely and friends

Photon heralds extend entanglement

Quantum Entanglement should be an incredibly useful tool for communications. Though it won’t ever become an ansible, or even the basis for subspace radio, the ability to communicate through entangled pairs of quantum particles would, in theory, create a situation where no third party could intercept the message. Which means that we’d finally have unbreakable secure communications. In theory at least. There are a number of practical problems.

One of the problems is that it’s very difficult to extend entanglement to a useful range. Generally anything over a meter is pretty much impossible and typical length scales are a million times shorter than that. But there’s some hope that the range can be extended much much further by using a series of “repeaters”, not unlike the way telecommunications or ethernet systems manage to extend their range to world spanning distances.

From Ars Technica:

“Researchers in Geneva, Switzerland have now built a possible model for a quantum repeater, using entangled photons to excite rare-earth atoms embedded in two crystals. The atoms themselves have correlated quantum states, and when they emit new photons, those are also entangled, guaranteeing that the original “message” is passed along. Devices built along these lines could act as solid-state nodes within quantum networks, allowing for larger quantum computing systems.

As with many other successful entanglement experiments, the core of the Swiss apparatus is a type of crystal that absorbs one photon and then emits two that have opposite polarization states. The specific state of each of these photons is undetermined until measured. But because they are entangled—correlated—measuring the polarization of one photon instantly reveals the state of the second, no matter how far they are separated in space.

(No information can be transmitted this way since the people on either end would need to discuss how the measurement should be taken, and that discussion takes place at light-speed.)

[…]The absorption-remission process and a photon passing through without interaction both may trigger a detector, but the wrong photon means the message isn’t actually received. To ensure that only the right photons are counted in the repeater, Imam Usmani and colleagues prepared the initial signal so that each “proper” photon is partnered with a second photon of a different wavelength known as a herald. “

Neat idea. There are any number of simple useful applications for such a tool. Credit card and financial transactions lead the list and are probably what are going to bring the big money to solving this problem.

Plus you gotta love any headline that lets you glom together the idea of photon heralds with quantum entanglement. Grin.

(Remember that none of this sort of behavior is allowed in a deterministic system. There’s no right and wrong answer in the way that a reductionist world view would allow here. And that’s useful to keep in mind when one is doing theology, especially if one has been reading a great deal of scholastic theology.)

I wish entanglement would be the basis of a working ansible, but no, people much smarter than I have thought it all the way through. While the entangled particles will act instantaneously, the ability to extract information from the action is limited to the speed of light.

There’s a similar distinction made in when thinking about phase velocity in a wave packet – it’s easy to construct a thought experiment where the phase velocity is greater than the speed of light, but there’s no information being transmitted. (drat).